1. Field of the Invention
The present invention relates to a method of reducing the roughness of a thick insulator layer deposited on a substrate intended for use in the electronics, optoelectronics, or optics fields. The invention also relates to a method of bonding and transferring layers using the above-specified method. The invention is of particular application in the production of composite “substrate on insulator” type substrates.
2. Description of Related Art
Substrates known by the acronym “SOI” (substrate on insulator) correspond to substrates in which an insulator layer, such as silicon dioxide (SiO2), is interposed between a support layer of silicon and a surface layer of silicon. Insulators can include oxides, nitrides, and oxynitrides.
One of the steps for producing an SOI involves bonding a “receiver” substrate onto the insulator layer. “Bonding” as used here is understood to mean bonding by molecular bonding, wherein two perfectly flat surfaces adhere to each other without the application of an adhesive, this being possible at ambient temperature. The quality of the bond obtained is characterized in particular by the bonding energy, which is defined as the binding force existing between the two layers bonded together. The quality of the bond may be improved by carrying out a suitable treatment of the surfaces to be bonded in order to provide a smoother bonding surface.
A technique for forming these substrates is known to persons in the art as “LPCVD TEOS,” or “low pressure chemical vapor deposition—tetraethylorthosilicate.” This involves depositing a layer of silicon dioxide on a support layer using a low pressure vapor phase deposition technique with tetraethylorthosilicate as the source material. This method enjoys a number of advantages with respect to the uniformity and density of the oxide layer obtained, and does not consume the substrate layer on which the formed silicon oxide lies. Oxides formed by a thermal oxidation technique often consume the substrate layer on which the formed oxide lies.
One of the disadvantages of the LPCVD TEOS technique, however, is that the layers of silicon dioxide deposited are significantly rougher than layers formed by thermal oxidation. For example, the surface roughness of a 150 nm (nanometer) thick TEOS layer may typically be more than 5 Å (angstroms) rms over scan widths of 1 μm (micrometer) by 1 μm, in contrast to a layer formed by thermal oxidation which typically may be about 1.50 Å rms.
Other deposition techniques are also known, such as LPCVD using silane (SiH4) as a precursor, or plasma etch chemical vapor deposition (PCVD), for example. Reference in this regard should be made to the article by Wolf and Tauber, “Chemical vapor deposition of amorphous and polycrystalline films,” Silicon processing for the VLSI era, Vol 1, pp 189-207, Method Technology. Unfortunately, these deposition techniques also lead to the production of insulator layers with very high roughness. Further, the roughness increases with the thickness of the deposited layer.
For that reason, production of a thick (i.e. more than 20 nm) insulator layer—typical for the fabrication of SOI type products—generally results in a degree of roughness which is incompatible with the constraints imposed by very high quality molecular bonding. Ideally, the roughness is preferably less than 5 Å rms to allow bonding, or even less than 2 Å rms over scan widths of 1 μm by 1 μm, in the context of a layer transfer application, known as the SMART-CUT® method.
The technique of subjecting a substrate to a plasma to modify the structure of the surface layer of the substrate is known in the art. “Plasma treatment” of a surface for bonding is defined as exposing that surface to a gas plasma—in particular under vacuum or at atmospheric pressure—prior to bringing the surfaces to be bonded into contact. The treatment is carried out by controlling various exposure parameters such as the nature and flow rate or pressure of the gas supplied to the chamber inside which the operation is carried out, as well as the power density. Two types of treatment can be distinguished: the first, termed “activation plasma,” is known to promote the bonding energy between two layers. The second, termed “smoothing plasma,” is intended to reduce the roughness of the surface of the treated layer. The plasma operation parameters, in particular energy, are different in the two cases.
An article by D. M. Hansen et al., “Chemical role of oxygen plasma in wafer bonding using borosilicate glasses,” Applied Physics Letters, Volume 79, Dumber 21, 19 Nov. 2001, discloses a method of plasma activation of a thin borosilicate layer, deposited by LPCVD. Borosilicate type glasses are alloys of boron trioxide (B2O3) and silicon dioxide (SiO2). The experiment reported in that article concerned the treatment of a borosilicate layer of about 30 Å (3 nm) with an oxygen plasma in RIE (reactive ion etching) mode, for five minutes, at 0.6 W/cm2 (watts/square centimeter), with a pressure inside the chamber of 30 mTorr (1 mTorr=1.33×10−1 Pa). The results obtained reveal an improvement in bonding and showed that the roughness of the treated surface was not affected by the activation plasma treatment.
D. Pasquariello et al., in an article entitled “Surface energy as a function of self-bias voltage in oxygen plasma wafer bonding,” Sensors and Actuators 82 (2000) 239-244, studied the influence of the kinetic energy of ions of an oxygen plasma on the bonding energy of silicon wafers. The tests showed that a silicon wafer having an initial roughness of 0.9 Å rms could achieve a roughness of close to 0.60 Å rms over 1×1 μm2. The author also concluded that the kinetic energy of plasma ions had no influence whatsoever on the smoothing quality of substrates treated therewith.
Further, an article by H. Moriceau et al., “Interest of a short plasma treatment to achieve high quality Si—SiO2—Si bonded structures,” Abstract no. 1006, ECS 2003, showed the smoothing effect of a plasma on thermal oxides of SiO2 with an initial roughness which was, however, fairly low (2.3 Å rms over 0.5×0.5 μm2 to 20×20 μm2). It was shown that fairly long exposure times to a plasma increased its smoothing effect.
While these articles show an improvement in the roughness of the surface of the substrate, those observed improvements occurred using films which started with very low roughness. Thus, there remains a need to eliminate the roughness of a thick insulator layer which is not to be or cannot be formed by oxidation of its starting substrate, and which has a high initial roughness. The present invention now satisfies this need.